Answer : The vapor pressure of hexane = 140.5055 torr
Solution : Given,
Concentration of Naphthalene by mass = 10 %
Vapor pressure of pure Hexane = 151 torr
From the periodic table, the molar masses of Naphthalene and Hexane are:
Molar mass of Naphthalene = 128 g/mole
Molar mass of Hexane = 86 g/mole
From the Raoult's law, the vapor pressure of the component is equal to the vapor pressure of the pure component multiplied by its mole fraction.
Formula used :
..............(1)
Where,
= vapor pressure of component i
= vapor pressure of pure component i
= mole fraction of of component i in the solution
For this problem, few steps are involved:
step 1 : concentration of 10% naphthalene by mass means 10 g of naphthalene present in 100 g of solution.
This means,
The mass of naphthalene = 10 g
The mass of hexane = 100 - 10 = 90 g
step 2 : find the number of moles of naphthalene and hexane.
Formula used :
Number of moles =
Moles of Naphthalene = = 0.0781 moles
Moles of Hexane = = 1.0465 moles
Total moles = Moles of Naphthalene + Moles of Hexane
= 0.0781 + 1.0465 = 1.1246 moles
step 3 : Now, calculate mole fraction of Hexane
Mole fraction of Hexane = =
= 0.9305
step 4 : Now, find the vapor pressure of Hexane by using above formula (1).
Now put all the values in this formula, we get
Vapor pressure of Hexane = Vapor pressure of pure Hexane × Mole fraction of Hexane
= 151 torr × 0.9305 = 140.5055 torr